The invention relates to tape storage devices, and in particular, to a method of detecting and correcting for slippage of tape media in a tape storage device.
Tape storage devices have been widely employed in industry for over thirty years due to their ability to store large amounts of data on a relatively small inexpensive removable format. Tape storage devices are available in a variety of designs, but in all cases, magnetic tape media is wound between a pair of tape reels, known in the art as a supply reel and a take-up reel, as data records are exchanged. It is important in data storage to efficiently organize large quantities of data records on a single tape media without overwriting existing data records. During operation, tape storage devices must be able to precisely determine the location and position of the tape media at all times. To accomplish this task, various methods have been employed. One method is to write special identifying marks on the tape media using the same read/write head or heads that are used to record the data records. The recording head then references the various identifying marks to determine the location and position of the tape media. These identifying marks are placed at regular intervals or where one data record ends and another begins.
Unfortunately, when the tape media is wound at high speeds, the read/write head cannot keep up with the winding operation to determine location. In this case, an electromechanical device is used to track the distance the tape media travels with reference to the beginning of the tape (xe2x80x9cBOTxe2x80x9d). One such device is a rotation sensor that counts revolutions of a rotating magnet in one or both of the tape reel drive motors. The rotation sensor outputs pulses that are counted to provide information indicative of the length of tape that is wound on or off of a tape reel. Unfortunately, because they detect rotation of the tape reel motor magnet, rotation sensors typically have a limited resolution of 8 to 16 pulses per motor revolution.
Another device used to track the distance the tape media travels with reference to the BOT is a tachometer. The tachometer is connected to one of the guide rollers in the tape path and provides much higher resolution than rotation sensors. Since the guide rollers are driven directly by frictional contact with the tape media, any count of the rotational position is directly proportional to the distance along the tape media from the BOT. Similar to rotation sensors, tachometer sensors output pulses that are counted to provide information indicative of the length of tape that has passed over a guide roller. The tachometer information is mechanically derived and its accuracy depends upon maintaining a constant relationship between the number of tachometer pulses generated and the actual longitudinal length of tape transported past the magnetic head location.
Tachometers and rotation sensors, however, produce inaccurate results if slippage occurs in the tape transport device. Slippage can occur at a variety of interfaces. One well recognized slippage, known in the art as xe2x80x9ctach slip,xe2x80x9d results when the tape media slips relative to a guide roller that is driving the tachometer. Tach slip can occur suddenly and drastically when the tape media is accelerated or stopped, especially if tension control is poor or the roller is contaminated with dirt and dust. In addition, tach slip can occur gradually over a long distance, eventually building into a significant error. This phenomenon is known in the art as xe2x80x9ccreepxe2x80x9d.
Another well recognized slippage, known in the art as xe2x80x9cinterlayer slip,xe2x80x9d occurs between the layers of the tape media wound around a tape reel. Both Interlayer slip and tach slip can occur gradually, or suddenly and drastically. In some cases, the slippage is so severe that tape layers become folded back across one another, a phenomenon known in the art as a xe2x80x9cZ fold.xe2x80x9d
Regardless of the cause, slippage results in inaccurate knowledge of tape position. This in turn leads to an inability to find data on the tape media or to more serious situations such as running the tape media off the reel completely.
The present invention overcomes the problems outlined above and advances the art by providing a method for detecting and re-calibrating a position sensing assembly in a tape storage device to account for slippage so that the location and position of the tape media is known at all times. In a first example of the present invention, the method comprises sensing a position of one of a supply reel and a take-up reel. In response to sensing the position of the supply reel or the take-up reel, a guide roller position is sensed. The guide roller position corresponds to the position of the guide roller when the position of the one of the supply reel or the take-up reel is sensed. In response to sensing the position of the guide roller, the tape media is wound between the supply reel and the take-up reel. In response to winding the tape media, a second position of the one of the supply reel and the take-up reel is sensed. In response to sensing the second position of the one of the supply reel and the take-up reel, a second position of the guide roller is sensed. The second position of the guide roller corresponds to the position of the guide roller when the second position of the one of the supply reel or the take-up reel is sensed. In response to sensing the second position of the guide roller, a position difference between the first position of the guide roller and the second position of the guide roller is determined. The position difference is compared to a previous position difference to determine a relative position difference, which is used to detect slippage.
Because the tape media is relatively thin, the position difference is relatively small where no slip occurs. if slippage occurs, the difference in position is significantly larger in absolute value terms because it will include the normal difference in position of the guide roller 109 and an amount of slippage. If slippage is detected, a correction is made in the tape storace device to account for the amount of slippage.
In another example of the present invention, a position difference is determined for both the take-up reel and the supply reel to more accurately detect slippage. In this example, a first position of one of the supply reel and the take-up reel is sensed. Substantially simultaneously, a first corresponding position of the guide roller is sensed. In response to sensing the position of the guide roller, a first position of the other one of the supply reel and the take-up reel is sensed. Substantially simultaneously, a second corresponding position of the guide roller is sensed. In response to sensing the second position of the guide roller, the tape media is wound between the supply reel and the take-up reel. In response to winding the tape media, a second position of the first one of the supply reel and the take-up reel to complete one full revolution is sensed. Substantially simultaneously, a third corresponding position of the guide roller is sensed. In response to sensing the third corresponding position of the guide roller, a relative position difference is determined for the first one of the supply reel and the take-up reel to complete one full revolution. The first position difference is used as described above to determine if slippage occurred. In response to determining that slippage occurred, a correction is made in the tape storage device to account for the amount of slippage. In response to determining that slippage did not occur, a second position of the other one of the supply reel and the take-up reel to complete one full revolution is sensed. Substantially simultaneously, a fourth corresponding position of the guide roller is sensed. In response to sensing the fourth position of the guide roller, a relative position difference is determined for the other one of the supply reel and the take-up reel. The relative position difference is also used to determine if slippage occurred. In response to determining that slippage did occur, a correction is made in the tape storage device to account for the amount of slippage. In response to determining that slippage did not occur, the position change of the supply reel and the take-up reel is continually monitored for slippage.
In the context of the present invention, the first, second, third, fourth, etc. connotations used to reference the tape storage device elements and positions are used for the purpose of differentiating between different elements and positions and are not used to indicate a specific sensing sequence.